* remove unused function to interpolate URLs

* share function to add headers between ds/plugin proxies

* stop performing unnecessary plugin setting lookup

* fix bug causing runtime errors when using complex templated URLs

* lower case util functions not used outside of pluginproxy package

* change test URL to a (valid) dummy URL to make intent clearer

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>